Lined Chiton / Woody Chiton / Butterfly Shell (Internal Valve)
Phylum: Mollusca, Class: Polyplacophora, Order: Chitonida, Family: Tonicellidae, Genus: Tonicella, Species: lineata · Tonicellidae (Chiton family) · Chiton (Internal Intermediate Valve). This is an individual plate, not the whole animal. Shape is butterfly-like or saddle-shaped.
Species
Tonicella lineata (Confidence level: 85%)
Shell Type
Chiton (Internal Intermediate Valve). This is an individual plate, not the whole animal. Shape is butterfly-like or saddle-shaped.
Family
Tonicellidae (Chiton family)
Size
Approximately 12-18mm in width. This represents a single valve from an adult specimen that likely reached 1.5 to 2 inches in total length.
Color & Pattern
Base color is off-white to chalky grey due to beach weathering. Distinctive dark zig-zagging transverse bands or 'lines' are visible across the lateral areas. The apex area appears sun-bleached.
Rarity
Common (as live animals), but finding individual weathered 'butterfly' valves washed up on sand is considerered 'Uncommon' to 'Fairly Common' depending on local wave action.
Habitat
Marine intertidal and subtidal zones. Usually found clinging to rocks heavily encrusted with coralline algae, which these chitons graze upon.
Geographic Range
Common along the North Pacific coast, ranging from the Aleutian Islands, Alaska, down to Southern California.
Description
A single intermediate valve of a Chiton. Often called 'butterfly shells' by beachcombers due to their winged shape, this specimen features two lateral areas separated by a central jugal tract. The surface is sculpted with fine growth lines and decorated with characteristic dark, wavy concentric stripes.
Key Features
Saddle-shaped geometry; wavy, dark zig-zag lines; presence of a central smooth ridge (jugum); two distinct lateral 'wings' where the plate would insert into the chiton's fleshy girdle.
Collector Value
Low monetary value ($1-$5), but high aesthetic/educational value. They are popular for 'shell art' and used by hobbyists to create 'butterfly' displays.
Condition Notes
Fair to Good. The plate is intact but shows significant 'beach wear'—the edges are slightly rounded and the natural vibrant gloss of a live specimen has been replaced by a matte, chalky texture from salt and sand abrasion.
Interesting Facts
Chitons have eight interlocking plates that allow them to roll into a ball like an armadillo if dislodged. Their teeth are capped with magnetite, a hard magnetic mineral, allowing them to scrape algae off solid rock effortlessly.
Ecological Role
Important grazers in rocky intertidal ecosystems. They control algal growth and are a food source for sea stars, crabs, and certain shorebirds. They are not currently considered a threatened species.
Similar Species
Mopalia muscosa (Mossy Chiton) valves, which are usually larger and lack the fine zig-zag lines, and Tonicella lokii, which has much more intricate, thinner line patterns.
Beach Finding Tips
Search in the 'wrack line' or small tide pools after a heavy storm. Because these valves are light, they often get caught in sea foam or deposited in fine shell hash near rocky outcroppings.